This subject fascinates me, and thanks, manhattangirl, for bringing it up. And most interesting, what everyone else has also posted. Just a few thoughts from me, just because I love talking about Ennis and Jack...
This "delicate balancing act," as aintfoolin said, sure is at the heart of what kept these two together during all these years. I do have sympathy for Ennis, but if I were in Jack's situation, I wouldn't have put up with it. I say that, but I guess I would have put up with it if I were like Jack, because Jack's love was such that it was worth it for him, and he was a dreamer after all, always believing that one day Ennis would come around.
They were adrift with this relationship, because they had no real models to go by. They weren't doing the usual thing of the man and woman in those times (which were changing, but not that fast, and not so much in their part of the country) -- where the man would basically run the show, at least on the surface, where it mattered to society (yet women know that they really have the power...and that power is withholding sex – but that's another huge subject).
So Ennis and Jack had to muddle through, and as it happens, their personalities tended to make one more dominant than the other. We do see Ennis's dominance – his defining how it was going to be – as mostly based on his fears, but as others have said, it's intrinsic in his personality for it to be this way, whoever he was with. And then it happens that Jack is willing to go along with it, for his own reasons.
What I just said has to do with their not having a life together, as Jack wanted, and with the terms on which they got together every so often, which were mostly up to Ennis. But I think on the basis of the time they spent together, it was a more equal partnership. I like to think that one reason why Ennis loved being with Jack so much was that he could give up the control that held him to tightly wound in the rest of his life, give up the dominance. Not that he was really aware of being "dominant" – it was just his way. Though if he had been able to have the sweet life with Jack, he would have realized he didn't have to be that way, that he was free to become a person free from the definitions of his youth.
And what tpe said is interesting, that each thought the other had more control. It just really is not so clear-cut, and that's what makes us continue to want to try to understand.
And this leads me to why I have special feelings for same-sex relationships, because they do have to make up their own rules, instead of doing the proscribed thing. (Not that many M/F couples do the proscribed thing so much any more these days.) The dynamics between Ennis and Jack would be different in our current times, but there still would be some power struggles in some sense, as there always are between people. But they would feel more free to figure out how to work them out.
I don't know if all this sounds like it has to do with the subject of "dominance," but it does to me. And I love everyone else's thoughts.
kathy